Silicon photonics (SiPh) has many potential applications in telecommunications systems due to low fabrication cost, leveraging of existing CMOS technology, and compactness. Currently, signal transceivers are a primary commercial application for silicon photonics devices. Typical applications require or would benefit from a low fiber-to-chip insertion loss. Polarization independence is also a requirement for many applications.
Nanophotonic silicon waveguides typically have a high birefringence. Active devices such as p-i-n junction based phase shifters and modulators also have polarization dependence that stems from the difference in overlap (or confinement) of the so-called Transverse Electric (TE) and Transverse Magnetic (TM) optical modes within the silicon cores. Polarization dependence can be problematic as different portions of the same optical signal are subject to different conditions.
Polarization dependence of silicon nanophotonic circuits can effectively be addressed through what is known as “polarization diversity”. In this approach, both polarization components of optical signals are separated shortly after entry into the silicon optical chip, with one of the components converted to the same polarization as the other. The two components are then processed by two identical but separate circuits, after which one of the components is converted back to the orthogonal polarization and combined with the other upon exiting the silicon optical chip. In some implementations of polarization diversity, the two components are processed in the same photonic circuit, but in reverse directions. In any case, the polarization splitters and converters required to implement polarization diversity introduce some impairments to the optical signals, such as loss, polarization dependent loss, and polarization crosstalk. This and the duplication of the photonic circuits, if it is employed, increases the size, complexity, and cost of these photonic devices.
Variable optical attenuators (VOAs) are a useful general-purpose optical component that can be used for channel equalization or modulation, typically at a lower frequency than dedicated high-speed modulators. A VOA can be implemented using a p-i-n junction in which the optical signal can be controllably attenuated by the injection of carriers (e.g. via an electrical current applied to the junction). Another VOA implementation utilizes an integrated Mach-Zehnder interferometer. Such a modulator is driven by an optical phase change which can be driven using a p-i-n junction or a locally heated waveguide section, typically referred to as a thermo-optic phase shifter.
Micron-scale silicon waveguides can yield polarization independent devices. However, carrier injection devices in such platforms are currently limited in modulation frequency, to a maximum of approximately 1 MHz.
Therefore there is a need for a photonic device for adjusting (e.g. attenuating, phase-shifting, or both) optical signals, that is not subject to one or more limitations of the prior art.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.